This invention relates to a photothermographic image forming element, and more particularly, to a photothermographic image forming element suitable for use in a photomechanical process and especially adapted for scanners and image setters. More specifically, it relates to such a photothermographic element featuring low Dmin and consistent manufacture.
There are known a number of photosensitive elements having a photosensitive layer on a support wherein images are formed by imagewise exposure. Among these, a technique of forming images through heat development is known as a system capable of simplifying image forming means and contributing to the environmental protection.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the photomechanical process field to reduce the quantity of spent solution. Needed in this regard is a technology relating toxe2x80x94photothermographic elements for use in photomechanical process which can be effectively exposed by means of laser scanners or laser image setters and produce distinct black images having a high resolution and sharpness. These photothermographic elements offer to the customer a simple thermographic system that eliminates a need for solution type chemical agents and is not detrimental to the environment.
The technology of forming images through heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d in xe2x80x9cImaging Processes and Materials,xe2x80x9d Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic elements generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an organic binder matrix. Photothermographic elements are stable at room temperature. When they are heated at an elevated temperature (e.g., 80xc2x0 C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the reducible silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
For the preparation of organic acid silver salts, it is well known in the art to treat an organic acid such as an aliphatic carboxylic acid with an alkali to form the organic acid alkali metal salt, and add silver nitrate thereto, whereupon exchange reaction takes place between the alkali metal of the organic acid alkali metal salt and silver ion to thereby form the organic acid silver salt. This process, however, is difficult to obtain homogeneous organic acid silver because the organic acid alkali metal salt becomes solid, which hinders uniform proceeding of exchange reaction with silver ion.
EP 762,196 and JP-A 90550/1997 disclose to introduce metal ions or metal complex ions belonging to Group VII or VIII (Groups 7 to 10) in the Periodic Table into photosensitive silver halide grains to be used in thermographic image forming elements and to introduce hydrazine derivatives into photosensitive elements to achieve high contrast photographic properties. Since the photosensitive elements containing such nucleating agents as hydrazine derivatives tend to fog, there is a desire to produce high contrast images while suppressing the fog.
Therefore, an object of the invention is to provide a photothermographic image forming element suitable for use in a photomechanical process and exhibiting excellent photographic properties including a high contrast and low fog so that it may especially comply with scanners and image setters.
According to the invention, there is provided a photothermographic image forming element comprising a non-photosensitive silver salt and a photosensitive silver halide on a support. The non-photosensitive silver salt has been formed by simultaneously adding a previously prepared aqueous solution of silver nitrate and a previously prepared solution or suspension of an organic acid alkali metal salt to a reactor and mixing them therein. A photosensitive layer containing the photosensitive silver halide or a layer disposed adjacent thereto or both contain a nucleating agent.
Preferably, the solution or suspension of an organic acid alkali metal salt contains a tertiary alcohol.
Also preferably, the nucleating agent is a substituted alkene derivative of the following formula (1), a substituted isoxazole derivative of the following formula (2), an acetal compound of the following formula (3), or a hydrazine derivative. 
R1, R2, and R3 are independently hydrogen or substituents, and Z is an electron attractive group or silyl group, and at least one pair of R1 and Z, R2 and R3, R1 and R2, and R3 and Z, taken together, may form a cyclic structure; 
R4 is a substituent; 
X and Y are independently hydrogen or substituents, A and B are independently alkoxy, alkylthio, alkylamino, aryloxy, arylthio, anilino, heterocyclic oxy, heterocyclic thio, or heterocyclic amino groups, or X and Y, and A and B, taken together, may form a cyclic structure.
In one preferred embodiment, the photosensitive layer has been formed by applying a coating solution containing the photosensitive silver halide and a binder in a solvent in which water constitutes at least 60% by weight of the solvent, the photosensitive silver halide has been formed independent from the non-photosensitive silver salt and added during preparation of the coating solution, and the binder contains at least 50% by weight of a polymer latex having a glass transition temperature of xe2x88x9230xc2x0 C. to 40xc2x0 C.